Known pistons are usually designed cylindrical in a region of the piston crown in order to be able to close off the space for the combustion taking place in a combustion engine. For weight reduction of the piston, the piston hubs for receiving a piston pin are recessed to the inside with respect to the diameter of the piston skirt. Because of this circumstance, material can be saved in the outer regions and a weight reduction achieved by this.
In general, however, even a piston of this type with pin hubs relocated to the inside, has to have skirt wall sections at the height of these pin hubs as well as below these on a part of the circumference, with which it contacts the cylinder wall of the combustion engine after the installation in the cylinder, supporting itself on said cylinder wall. These skirt wall sections in this case are also called supporting skirt wall sections on the pressure and back pressure side, since these are responsible for guiding the piston in the cylinder. The skirt wall sections in this case prevent in particular undesirable tilting about the pin axis in the upper and lower dead centre. The two skirt wall sections or the two skirt walls are connected via box walls, which include the piston pin hubs.
From EP 0 838 587 B2. a piston with skirt walls and box walls connecting these is known, which follow the course of the edges of the skirt walls running in the direction of the piston axis. Here, the width of the two skirt walls is designed growing larger towards the lower end of the piston skirt, as a result of which the two box walls are thus embodied and are designed inclined towards each other in such a manner that their spacing in a lower region is greater than in an upper region. In other words, the connecting walls thus run conically to each other as it were. Because of this, additional material can be saved in particular in those zones, in which the skirt walls can be designed narrower.
From EP 1 348 859 B2. a further piston with sloping box walls is known.
Furthermore, from DE 10 2006 020 861 B4. a single-stage forging method for producing bush-like forged parts in a mould, wherein a pre-pressing blank is placed in a single die impression is known. The die impression in this case comprises an outer die ring, a lower die mandrel through which a lower ejector can be actuated and an upper die mandrel, through which an upper ejector can be actuated, wherein in the impression of the upper die mandrel a negative extraction cone is provided. Following this, the die is closed and the pre-pressing blank is moulded by pressing the die mandrels against each other, wherein through a combined actuation of the ejectors and simultaneous opening of the die an ejection of the finish-forged piston can take place. The method according to the invention serves in particular to extend the tool life of the dies.